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Isotopes
What is an isotope?
• Not every atom is exactly the same
• We know that different elements have
different number of protons.
• However even atoms of the same element
are not exactly the same.
• Atoms of the same element can have
different number of neutrons. These are
called Isotopes.
It’s all about the Mass Number
• Because Isotopes differ in their number of
neutrons, the mass of the atom also
changes.
• Isotopes are identified by their mass
numbers.
• Remember, mass number is the sum of
what two particles?
• Protons and Neutrons!
• 1 Proton
• 0 Neutrons
• Mass # = 1
1 Proton
1 Neutron
Mass # = 2
1 Proton
2 Neutrons
Mass # = 3
Isotopic Notation
• Isotopic Notation is generally
how isotopes are written.
• Mass number on top
• Atomic number on bottom
• This format makes it easy to
calculate number of Neutrons
• How do we calculate
neutrons?
• Isotopes have different mass numbers and
also different numbers of neutrons.
• Mass Number – Atomic Number = Neutrons
You Try! - Part 3
• Input your answers in this order, no spaces. It
should end up as one really big number!!!!!
Protons, then Neutrons, and then Electrons
• A) Carbon-12
• B) Carbon-14
• C) 104Be
• D) Phosphorus-32
D. Average Atomic Mass
• Weighted average of all isotopes based
on their % abundance.
• To determine, multiply the mass of the
isotope by the decimal form of its
abundance.
Average Atomic Mass =
(Mass * decimal) + (Mass * decimal)
D. Average Atomic Mass
• EX: Calculate the avg. atomic mass of oxygen if
its abundance in nature is 99.76% 16O, 0.04%
17O, and 0.20% 18O.
Average Atomic Mass =
(16*.9976) + (17*.0004) + (18*.0020) =
D. Average Atomic Mass
• EX: Find chlorine’s average atomic mass
if approximately 8 of every 10 atoms are
Chlorine-35 and 2 are Chlorine-37.
Avg.
Atomic
Mass
(35*.8) + (37*.2) = 35.4 amu
Ions
•
•
•
•
•
Atoms are identified by which particle?
PROTONS!
Different Isotopes have different?
NEUTRONS!
So, which particle do you think identifies
Ions???
• ELECTRONS!
Ions
• Ions form when an atom gains or
loses electrons.
• Why do they do that?
• Atoms gain or lose electrons in
order to reach a stable
configuration. We will talk more
about that in a few slides.
Ions
• Let’s look at a normal atom of Lithium.
• Protons?
•
OOO
• If the atom is neutral, how many
electrons?
•
o o o
• Can we ever change the number of
protons and still have a Lithium Atom?
•O O O
• What if the atom lost an electron to
become stable.
•o o
• Is the atom still neutral?
• What would be the charge of this
atom now?
• Can we ever change the number of
protons and still have a Lithium Atom?
•O O O
• What if the atom gained an extra electron
to become stable.
•o o o o
• Is the atom still neutral?
• What would be the charge of this
atom now?
In review….
• If an atom loses electrons, the overall
charge of the atom becomes positive.
Positive because there will be more
protons than electrons.
• If an atom gains extra electrons, the
overall charge of the atom becomes
negative. Negative because there will be
more electrons than protons.
So what if you see this?
• An atom of Nitrogen has a -3 charge.
• What happened?
• How many electrons does this ion have?
• An atom of Magnesium has a +2 charge.
What happened?
• How many electrons does this ion have?